Contactless charging technology allows contactless power transfer from a power source (for example a charger) to a load, for example a wireless communication appliance, for example a cellular mobile telephone, for the air space separating the charger and the load.
Among the various existing contactless charging standards, it is possible to cite the Qi standard developed by the “Wireless Power Consortium” (WPC), which is well known to a person skilled in the art, and notably the low-power version, which can deliver a power of up to 5 W.
For all intents and purposes, a person skilled in the art will be able to refer to the document entitled “System Description Wireless Power Transfer” volume I: Low Power, Part 1: Interface Definition, version 1.1.2, June 2013 (incorporated by reference).
Briefly, contactless power is transferred from a base station or transmitter to a receiver that is magnetically coupled to the transmitter by means of coils, for example.
The power transfer is based on a magnetic field generated by the transmitter and captured by the receiver.
The Qi standard allows various techniques (topologies) for generating the magnetic field in the transmitter. However, this magnetic field is always an oscillating field and the range of frequencies is between 110 kHz and 205 kHz.
The way in which the magnetic field is generated and the way in which the power varies depend on the topology used in the transmitter.
The receiver converts this magnetic field into an electrical potential that is used in order to charge the batteries or supply power to a device.
So as to be able to regulate and provide security for the system, a communication channel exists between the receiver and the transmitter.
The receiver measures the power that it receives and compares it with the power that it needs. The difference between these two power values is a regulation error.
This regulation error is transmitted to the transmitter by using the communication channel and the transmitter adapts the power of the transmitted field taking account of the received regulation error.
The communication from the receiver to the transmitter is based on modulation of the magnetic field resulting from a variation in the load at the terminals of the antenna of the receiver.
The transmitter can measure this load variation in order to deduce therefrom the transmitted bits and finally the transmitted message.
The transmitter conventionally comprises an inductive capacitive resonant system in which the inductive element is likewise the coil that generates the magnetic field. This resonant system is excited by a driving circuit (“driver”), and these together form a switching resonant circuit.
The modulation of the magnetic field that is performed by the receiver in order to communicate with the transmitter leads to a voltage variation at the terminals of the resonant circuit and to a variation in the current flowing in this resonant circuit.
Therefore, at present, a first solution in order to demodulate the receiver/transmitter communication channel consists in measuring this voltage variation of the resonant circuit.
However, such a solution requires numerous components and is sensitive to voltage noise from the receiver load.
Another solution consists in measuring the current flowing in the resonant circuit.
However, there again, such a solution requires numerous components, it is likewise sensitive to current noise from the receiver load, and the insertion of a current measuring interface decreases efficiency and can be critical to implement.